General Characteristics of f-block elements and extraction

The position of f block elements in the periodic table, is explained above.

The elements in which the extra electron enters ( n- 2 )f orbitals are called f- block elements. These elements are also called as inner transition elements because they form a transition series within the transition elements. The f-block elements are also known as rare earth elements. These are divided into two series.

i) The Lanthanide series (4f-block elements)

ii) The Actinide series (5f- block elements )

General Characteristics of f-block elements and extraction

The Lanthanide Series

The Lanthanide series include fifteen elements i.e. lanthanum (57 La) to lutetium (71 Lu). Lanthanum and Lutetium have no partly filled 4f- subshell but have electrons in 5d-subshell. Thus these elements should not be included in this series. However, all these elements closely resemble lanthanum and hence are considered together.

General properties of Lanthanides

1. Electronic configuration

The electronic configuration of Lanthanides are listed in the table 5.1. The fourteen electrons are filled in Ce to Lu with configuration [54 Xe ]4f1-14 5d1 6s2

2. Oxidation states

The common oxidation state exhibited by all the lanthanides is +3 (Ln3+) in aqueous solutions and in their solid compounds. Some elements exhibit +2 and +4 states as uncommon oxidation states.

La              -      +3

Ce              -      +3, +4, +2

Pr              -      +3, +4

Nd             -      +3, +4, +2

3. Radii of tripositive lanthanide ions

The size of M3+ ions decreases as we move through the lanthanides from lanthanum to lutetium. This steady decrease in ionic radii of M3+ cations in the lanthanide series is called Lanthanide contraction.

Cause of lanthanide contraction

The lanthanide contraction is due to the imperfect shielding of one 4f electron  by another in the same sub shell. As we move along the lanthanide series, the nuclear charge and the number of 4f electrons increase by one unit at each step. However, due to imperfect shielding, the effective nuclear charge increases causing a contraction in electron cloud of 4f-subshell.

Consequences of lanthanide contraction

Basicity of ions

i) Due to lanthanide contraction, the size of Ln3+ ions decreases regularly with increase in atomic number. According to Fajan's rule, decrease in size of Ln3+ ions increase the covalent character and decreases the basic character between Ln3+ and OH- ion in Ln(OH)3. Since the order of size of Ln3+ ions

are

La3+> Ce3+ ............... >Lu3+

ii)    There is regular decrease in their ionic radii.

iii)    Regular decrease in their tendency to act as reducing agent, with increase in atomic number.

iv) Due to lanthanide contraction, second and third rows of d-block transistion  elements are quite close in properties.

v) Due to lanthanide contraction, these elements occur together in natural minerals and are difficult to separate.

The Actinide Series (5f block elements)

In 1923 Neils Bohr postulated the existence of an actinide series analogous to the lanthanide series.

The fifteen elements from actinium to lawrencium constitute the actinide  series of the periodic table.

General Properties of Actinide Series

The general electronic configuration of actinides is [Rn] 5f0,1-14 6d0,1-2 7s2  where Rn stands for radon core.

2.    Oxidation states

These elements shows the oxidation states of +2, +3, +4, +5 and +6. Out of these, +4 oxidation state is most common state.

3.    Radii of M3+ and M4+ ions

The ionic radii of actinide elements decrease gradually as we move along the actinide series. The steady decrease in the ionic radii with increase in nuclear charge is called actinide contraction and is analogous to lanthanide contraction.

Cause of actinide contraction

Cause of actinide contraction is the imperfect shielding by 5f-electrons. As we proceed from one element to the next one in actinide series, the nuclear charge increases by +1 at each next element which is not compensated due to poor shielding effect of 5f orbitals due to their more diffuse shape. Hence as the atomic number increases, the inward pull experienced by 5f-electrons increase. Consequently steady decrease in size occurs in the actinide series.

Extraction of Lanthanides from Monazite sand

The method used for extraction of lanthanides from monazite sand consists of the steps which have been shown in flowsheet.

Flow sheet : Extraction of lanthanide from monazite sand

Individual lanthanides are separated by a suitable physical method. The anhydrous fluorides and chlorides are heated under argon atmosphere in presence of calcium at 1270 K to get the individual metal. The pure metal is obtained by heating the trifluorides of lanthanides in the presence of calcium and lithium.